Active matrix organic light-emitting diode (AMOLED) displays are among the hot spots in today's flat panel display research. The organic light-emitting diode (OLED) has advantages such as low energy consumption, low production cost, being self-luminous, a wide viewing angle and a fast response speed, as compared with the liquid crystal display (LCD). At present, OLED displays are starting to replace traditional LCD displays in the fields of mobile phone, personal digital assistant (PDA), digital camera and the like. Pixel driving circuit design is the core technology of an AMOLED display, and is of important research significance.
Unlike thin film transistor liquid crystal displays (TFT-LCDs) which use a stable voltage for brightness control, the OLED display requires a steady current to control the light emission since the OLED is a current-driven type of device. In the existing driving circuit with two transistors 10, 20 and one storage capacitor C (referring to FIG. 1), the driving current IOLED is a current generated by applying a voltage Vdata provided by a data line to the driving transistor 20 operating in a saturation region, which current drives the OLED to emit light. The driving current is calculated as IOLED=K (VGS−Vth)2, where VGS is a voltage across the gate and the source of the driving transistor, and Vth is a threshold voltage of the driving transistor. There is non-uniformity among the threshold voltages Vth of the driving TFTs (i.e., 20 in the figure) of the pixels due to the fabrication process and the aging of the devices. This leads to a variation among the currents flowing through the OLEDs of individual pixels, thus affecting the display effect of the entire image.